In air-jet weaving machines a directed air jet that conveys the weft thread in free flight through the shed is produced by means of air nozzles. A plurality of groups of air nozzles (main nozzles, relay nozzles) are for that purpose controlled staggered in time by pneumatic valves. The function of the main nozzles is to accelerate the weft thread. The relay nozzles guide the tip of the thread through the shed.
The air nozzles are usually controlled according to a predefined schedule. Correct setting of the activation instants is therein dependent on, inter alia, the material of which the thread is made, the air pressure, and the prevailing climatic conditions and is based frequently on experimental values obtained from weaving trials.
It is known how to measure the weft thread's launch and arrival. The weft thread's launch is therein defined as the instant at which the weaving machine's thread brake is released and compressed air applied to the main nozzles. The thread's arrival is measured by two optical sensors. Its flight is modeled very coarsely through linear interpolating between its launch and arrival, with a uniform straight-line motion generally being assumed. The resulting interpolated straight line is then shown in a “nozzle settings” user display. Highly incorrect settings of the air nozzles can be seen in said display and have to be adjusted manually by changing the parameters step by step.